A polyimide film has been widely used in various applications such as the electric/electronic device field and the semiconductor field, because it has excellent heat resistance, chemical resistance, mechanical strength, electric properties, dimensional stability and so on. For example, a copper-clad laminate wherein a copper foil is laminated on one side or both sides of a polyimide film is used for a flexible printed circuit board (FPC).
One example of the polyimide films suitable as films for FPC, for example, is a polyimide film prepared from an aromatic tetracarboxylic acid component comprising 3,3′,4,4′-biphenyltetracarboxylic dianhydride as the main component and an aromatic diamine component comprising p-phenylenediamine as the main component by thermal imidization (See Patent document 1, for example.).
Meanwhile, in general, a polyimide film may not have adequate adhesiveness. When a metal foil such as a copper foil is bonded onto a polyimide film via a heat-resistant adhesive such as an epoxy resin adhesive, the obtained laminate may not have sufficiently high peel strength.
For example, Patent document 1 discloses a process for producing a polyimide film wherein a surface treatment solution containing a heat-resistant surface treating agent (coupling agent) is applied to a surface of a self-supporting film (solidified film) of a polyimide precursor solution so as to improve adhesive properties of the obtained polyimide film. There is a need for a polyimide film having excellent adhesive properties, which may be produced without applying a coupling agent onto a self-supporting film as described in Patent document 1.
Patent document 2 discloses a process for producing a polyimide film, which allows improvements in adhesiveness of the obtained polyimide film, comprising steps of:
flow-casting a composition comprising a solution of a polyimide precursor in an organic solvent, and heating the composition to form a pre-film, which preferably has an imidization rate of 70% or higher and a volatile content of 40 wt % or less; and
heating the pre-film at a temperature of from 450° C. to 630° C., more preferably at a temperature of from 520° C. to 580° C., to provide a polyimide film.
As for the polyimide to which the invention is applied, in Examples, polyimide films were prepared by chemical cure, or a combination of chemical cure and thermal cure, from pyromellitic dianhydride as an aromatic tetracarboxylic dianhydride and 4,4′-diaminodiphenyl ether, or alternatively 4,4′-diaminodiphenyl ether and p-phenylenediamine, as an aromatic diamine. In addition, Patent document 2 teaches that a polyimide film, which is produced by a conventional method as an end-product, may be heated at a temperature of from 450° C. to 630° C.
An alkali treatment has been also proposed as a method for improving adhesive properties of a polyimide film. For example, Patent document 3 discloses a polyimide film, which has a surface treated with an alkaline aqueous solution containing a permanganate and a hydroxide such as potassium hydroxide and sodium hydroxide, and may be bonded to a metal foil via a polyimide adhesive with high adhesiveness. In Examples, a commercially available non-thermoplastic polyimide film was immersed in an aqueous solution containing potassium permanganate and sodium hydroxide at 75° C. for 5 min. In addition, as a means for treatment of a polyimide film surface with an alkaline aqueous solution, Patent document 3 teaches that an alkaline aqueous solution may be sprayed on a polyimide film by means of a sprayer or a shower.
Incidentally, Patent document 4 discloses a process for producing a polyimide film, comprising steps of:
forming a film by flow-casting from a solution containing a polyamic acid and a catalytic compound;
immersing the film in water preferably at a temperature of 5° C. to 100° C. for 30 min to 100 hr. so as to remove the remaining catalytic compound and the solvent from the film; and drying the film.